Potential Dependent Reorientation Controlling Activity of a Molecular Electrocatalyst.

The activity of molecular electrocatalysts depends on the interplay of electrolyte composition near the electrode surface, the composition and morphology of the electrode surface, and the electric field at the electrode-electrolyte interface. This interplay is challenging to study and often overlooked when assessing molecular catalyst activity. Here, we use surface specific vibrational sum frequency generation (VSFG) spectroscopy to study the solvent and potential dependent activation of Mo(bpy)(CO)4, a CO2 reduction catalyst, at a polycrystalline Au electrode. We find that the parent complex undergoes potential dependent reorientation at the electrode surface when a small amount of N-methyl-2-pyrrolidone (NMP) is present. This preactivates the complex, resulting in greater yields at less negative potentials, of the active electrocatalyst for CO2 reduction.

Monitoring interfacial electric fields at a hematite electrode during water oxidation.

To understand the mechanisms of water oxidation on materials such as hematite it is important that accurate measurements and models of the interfacial fields at the semiconductor liquid junction are developed. Here we demonstrate how electric field induced second harmonic generation (EFISHG) spectroscopy can be used to monitor the electric field across the space-charge and Helmholtz layers in a hematite electrode during water oxidation. We are able to identify the occurrence of Fermi level pinning at specific applied potentials which lead to a change in the Helmholtz potential. Through combined electrochemical and optical measurements we correlate these to the presence of surface trap states and the accumulation of holes (h+) during electrocatalysis. Despite the change in Helmholtz potential as h+ accumulate we find that a population model can be used to fit the electrocatalytic water oxidation kinetics with a transition between a first and third order regime with respect to hole concentration. Within these two regimes there are no changes in the rate constants for water oxidation, indicating that the rate determining step under these conditions does not involve electron/ion transfer, in-line with it being O-O bond formation.

Time-Resolved Raman Spectroscopy of Polaron Formation in a Polymer Photocatalyst.

Polymer photocatalysts are a synthetically diverse class of materials that can be used for the production of solar fuels such as H2, but the underlying mechanisms by which they operate are poorly understood. Time-resolved vibrational spectroscopy provides a powerful structure-specific probe of photogenerated species. Here we report the use of time-resolved resonance Raman (TR3) spectroscopy to study the formation of polaron pairs and electron polarons in one of the most active linear polymer photocatalysts for H2 production, poly(dibenzo[b,d]thiophene sulfone), P10. We identify that polaron-pair formation prior to thermalization of the initially generated excited states is an important pathway for the generation of long-lived photoelectrons.

Water oxidation intermediates on iridium oxide electrodes probed by in situ electrochemical SHINERS.

Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is applied to the study of a state-of-the-art water oxidation electrocatalyst, IrOx, during oxygen evolution. The excellent sensitivity allows for in situ detection of surface intermediate species during cyclic voltammetry. Features in the Raman spectrum are correlated with the redox behaviour of the electrode, demonstrating a way to study the mechanisms of electrocatalytic water splitting using equipment available in most laboratories.

Vibrational sum-frequency generation spectroscopy of electrode surfaces: studying the mechanisms of sustainable fuel generation and utilisation.

The electrocatalytic oxidation of water coupled to the reduction of carbon dioxide, to make carbon based products, or the reduction of protons to provide hydrogen, offers a sustainable route to generating useful fuels. However new improved electrocatalysts and electrode materials are needed for these reactions. Similarly fuel cells for fuel utilisation rely on precious metal electrodes and new lower-cost materials are needed. Developing efficient catalysts for sustainable fuel generation can be accelerated with an improved understanding of the underlying mechanisms. Herein, we present a perspective on the use of vibrational sum-frequency generation (VSFG) spectroscopy to study such electrocatalytic mechanisms. We briefly outline the basic principles of VSFG spectroscopy pertinent to the study of electrochemical interfaces. We then review the use of VSFG to study water at charged and electrode interfaces, relevant to the mechanisms of water oxidation, the mechanisms of alcohol oxidation and also molecular electrocatalysts for carbon dioxide reduction.